Examples of semiconductor devices include field effect transistors (FETs), light emitting diodes (LEDs), and the like. For LEDs, for example, Group III-V semiconductors made of compounds of Group III and Group V elements are used.
A Group III nitride semiconductor using Al, Ga, In, or the like as a Group III element and using N as a Group V element has a high melting point and a high dissociation pressure of nitrogen, which makes it difficult to perform bulk single crystal growth. Further, no inexpensive conductive single crystal substrates having large diameter are available; therefore, such a semiconductor is typically formed on a sapphire substrate.
However, the sapphire substrate has an insulation property; accordingly, electric current does not flow in the substrate. Therefore, a light emitting diode conventionally used to have a lateral structure in which electric current flows in a lateral direction. This structure is obtained by partially removing a semiconductor laminate formed by sequentially growing an n-type Group III nitride semiconductor layer, an active layer (light emitting layer), and a p-type Group III nitride semiconductor layer on a sapphire substrate, to expose the n-type Group III nitride semiconductor layer, and providing an n-type electrode and a p-type electrode on the exposed n-type Group III nitride semiconductor layer and the p-type Group III nitride semiconductor layer, respectively.
In response to this, in recent years, the following techniques for obtaining vertical structure LED chips have been studied. After forming a buffer layer made of a certain element other than a Group III element (for example. Al, Ga, or the like) on a sapphire substrate, a semiconductor laminate including a light-emitting layer is formed. The semiconductor laminate is supported by a conductive support, and then a sapphire substrate is separated (lifted-off) by selectively dissolving the buffer layer by chemical etching. The conductive support and the semiconductor laminate are sandwiched between a pair of electrodes. Note that a “buffer layer” herein is a buffer layer for epitaxial growth of a semiconductor laminate, which also serves as a lift-off layer for separating the semiconductor laminate from a sapphire substrate.
In manufacturing a III nitride semiconductor LED chip having such a structure, a chemical lift-off process is used in which process an epitaxial layer is separated from a sapphire substrate by etching a lift-off layer with a certain etchant. This can be described in other words as “a growth substrate is lifted-off from an epitaxial layer”.
A method using CrN as a lift-off layer has been reported (PTL 1). Patent Document 1 discloses a method of manufacturing vertically structured Group III nitride semiconductor LED chips, including the steps of forming a plurality of light emitting structures made of a Group III nitride on a sapphire substrate with a lift-off layer made of CrN provided therebetween; forming a conductive support for integrally supporting these light emitting structures by Cu plating; separating the substrate from the light emitting structures by removing the lift-off layer by supplying an etchant by way of through-holes provided in the conductive support; and cutting the conductive support between the light emitting structures to singulate the plurality of the light emitting structures to obtain LED chips.
Meanwhile, techniques of forming a lift-off layer of a material other than CrN are being developed. For example, Patent Document 2 discloses a technique of using a single metal layer made of Zr or Hf as a lift-off layer, whereas Patent Document 3 discloses a technique of using a scandium nitride (ScN) film as a lift-off layer. The reason for using a material other than CrN as a lift-off layer is to obtain LED chips emitting a light with a shorter wavelength than that of blue light (for example, a wavelength of 400 nm or less). Specifically, as the wavelength of light to be generated is shorter, the Al content x in an AlxGa1-xN layer of the nitride semiconductor element is required to be higher. The growth temperature of AlGaN having an Al content which exceeds about 30 at. % is higher than about 1050° C., the melting point of CrN. Accordingly, when an AlGaN layer having such a high Al content is grown on a CrN layer, CrN melts under a high temperature environment. As a result, CrN is unevenly distributed on the sapphire substrate, and the AlGaN layer is partially grown directly on the sapphire substrate. Thus, it becomes difficult to perform chemical lift-off. On the other hand, when a material described in PTL 2 or PTL 3 is used as a lift-off layer, growth of high Al content AlGaN having a high growth temperature does not cause the melt of the lift-off layer. Thus, LED chips emitting a light with a short wavelength can be obtained by removing the lift-off layer.